This Week in Science

Science  30 Mar 2007:
Vol. 315, Issue 5820, pp. 1764
  1. Key to Liver Regeneration


    The liver is one tissue in mammals that can regenerate. Passino et al. (p. 1853) now find that hepatocyte proliferation is controlled by the neurotrophin receptor p75NTR, known primarily for its role in neurons in regulation of survival, apoptosis, and neuronal regeneration. Mice lacking p75NTR showed impaired hepatocyte proliferation. p75NTR appeared to act on hepatic stellate cells (HSCs), which differentiate in response to p75NTR and then make growth factors and extracellular matrix that support proliferation of hepatocytes. Modulation of the effects of p75NTR on HSCs could thus provide a therapeutic target for management of liver disease.

  2. Building Smarter

    Building construction is still dominated by traditional materials, and the conservative nature of architects and builders is understandable—it is hard to judge how a material may survive and perform during the 20- to 150-year lifetime of a building. However, the need to improve the energy efficiency and self-diagnostic capabilities of the built environment, along with a desire to improve the quality and functionality found in our interior spaces, has driven the adoption of new materials. Fernández (p. 1807) reviews this evolution in building design and fabrication and discusses how partnerships with the materials science community may accelerate the design and adoption of new materials.

  3. Not Polarized Initially


    Gamma-ray bursts (GRBs) that last several seconds or longer are thought to arise from the deaths of massive stars. Light from GRBs may be polarized if there are aligned magnetic fields around the collapsing star or in magnetized jets that it generates. Some observations had detected polarized signals from GRBs hours after the burst started. Mundell et al. (p. 1822; see the Perspective by Covino; published online March 15) have looked for polarized optical light just two and a half minutes after the burst GRB 060418 went off with the robotic Liverpool Telescope on La Palma, Canary Islands, which automatically responded to notification of the burst. At this early time, the light emitted came from the initial fireball of the explosion. No polarization was seen, to a limit of less than 8%, which rules out models with large aligned magnetic fields.

  4. Heat Flow Below

    The heat flow between turbulent, flowing liquid metals in the outer core and viscous, slowly convecting silicate magma in the lower mantle can be determined by identifying deep regions of postperovskite, a high-density polymorph of the common mantle mineral perovskite. The depth at which the polymorph forms can be measured seismically, and comparisons with pressure-temperature predictions from lab measurements and theory yield the heat flow at the interface depth. Van der Hilst et al. (p. 1813; see the Perspective by Buffett) have applied a seismic method from oil exploration to map the core-mantle boundary over a large region beneath Central America. They identify several regions where the postperovskite phase transition is seen, including multiple crossings, and infer heat fluxes.

  5. Recovering Plastic Strain

    The deformation of nanostructured metals differs from that of more typical coarse-grained metals, but are there also differences in their recovery after deformation? Rajagopalan et al. (p. 1831) used a microelectro-mechanical systems device to measure the stress-strain behavior of thin aluminum strips with 65-nanometer (nm) grains. The plastic deformation that remained when the sample was unloaded was very substantially reduced by in situ heating for 7 minutes. Such recovery of plastic strain under zero load was not seen in a similarly sized sample with 200-nm columnar grains. The authors explain the effect by the backward motion of pinned dislocations upon stress release that was aided by thermal energy. These observations, which were also seen in gold samples, could be significant for developing more reliable nanostructures and investigating new deformation mechanisms in nanostructured materials.

  6. Wedges of Stability

    Ice-sheet instability and sea level may be connected by a positive feedback that could accelerate sea-level rise in ways previously unexpected. Two reports examine how sedimentation at the grounding line of ice streams, the most distal location where the ice rests on ground and beyond which it floats on water, might affect their stability (see the Perspective by Anderson). Anandakrishnan et al. (p. 1835, published online 1 March) use radar surveys to show that rapid sedimentation of glacial till transported by an Antarctic ice stream is now occurring, and that the sedimentary wedge that has formed at the grounding line resembles the structures that occur on the sea floor at numerous locations nearby that were formed during the retreat of the ice shelves since the Last Glacial Maximum. Alley et al. (p. 1838, published online 1 March) discuss how this process should affect the stability of ice sheets. Small changes in sea level are not expected to cause rapid retreat, with the implication that the rapid increases in ice loss that have been documented recently at the margins of the Antarctic and Greenland ice sheets are caused by dynamic responses to climate warming. However, large sea-level increases, in the range of tens of meters, could overwhelm the stabilizing effects of sedimentation.

  7. Targeting Transmembrane Domains

    Reagents such as antibodies that target specific proteins are useful in research and medicine. There are methods to engineer antibody-like molecules that target the soluble regions of proteins, but targeting transmembrane regions remains a challenge. Now Yin et al. (p. 1817) describe a computational method to design peptides that target specific transmembrane helices. Peptides were designed that were specific for each of two closely related integrins involved in cell adhesion.

  8. Two Ways to Segment an Insect


    In the Drosophila embryo, the bicoid morphogen serves as a maternal anterior determinant during anterior-posterior axis development; however, this transcription factor is not found among all insects. Brent et al. (p. 1841) compared the molecular mechanisms of development in the fruit fly Drosophila and the wasp Nasonia. Drosophila bicoid serves both an instructive function during anterior patterning, and a permissive function for the repression of trunk genes in the anterior region. However, in the wasp, these two functions remain discrete: Nasonia orthodenticle (otd) performs bicoid function for anterior development, while maternal giant represses trunk fate. Thus, in the wasp, otd and giant jointly accomplish bicoid's role in Drosophila.

  9. Big Fish, Little Fish, Shellfish

    The loss of large predators from ecosystems, often caused by human activities, can have effects that cascade through the rest of the food chain. Myers et al. (p. 1846) quantitatively assess the ecosystem consequences of the functional elimination of top predators from a northwest Atlantic marine environment. The loss of 11 species of large sharks, with numerical declines during a 35-year period of up to >99%, resulted in population increases in 12 out of 13 species of smaller sharks, rays, and skates eaten almost exclusively by large sharks. One of these, the cownose ray, has increased 20-fold since 1970. Its prey, bivalve mollusks, have been reduced to levels where commercial shellfisheries have suffered and where the water-filtering service of their remnant populations has been compromised.

  10. A Fly Model of PolyQ Disease

    More than 40 human diseases are known to be caused by the expansion of simple repeat sequences, the majority being trinucleotide repeats such as CAG or CGG. However, few models for repeat instability recapitulate the striking features seen in human patients, and few or no therapeutics to clamp repeat instability. Jung et al. (p. 1857, published online 1 March; see the Perspective by Fortini), in the model organism Drosophila, observe striking CAG repeat instability that recapitulates several key features of human disease, including large repeat expansions, with repeat size variations similar to that of human patients. The pathologic CAG/polyglutamine (polyQ) protein, encoded by the expanded CAG repeat, enhanced repeat instability through an inhibitory effect on a regulatory protein involved in DNA repair and replication.

  11. Attention and Information Flow

    Cortical neurons modulate their activity with shifts in attention, but the source and flow of attention signals are unclear. Buschman et al. (p. 1860) used 50 electrodes to record simultaneously the activity from three cortical regions thought to be critical for attention. Bottom-up shifts of attention were first reflected in the parietal cortex, whereas top-down shifts of attention were reflected first in the frontal cortex. Thus, external control of visual attention originates in parietal cortex, but internal control of visual attention is directed from the frontal cortex.

  12. Imaging with Unscattered Electrons

    In scanning tunneling microscopy (STM) of organic molecules on surfaces, contrast is achieved because the molecules change the height and width of the barrier through which low-energy electrons from the tip must tunnel. Bannani et al. (p. 1824) present on alternative approach, ballistic electron emission microscopy, in which molecules are adsorbed on a metal film grown on semiconductor substrate. This method is complementary to STM in that the ballistic current collected from the substrate represents electrons that traversed the molecules without scattering. The authors present images for C60 and a planar conjugated organic molecule and show that by varying the electron energy, information about unoccupied molecular orbitals can be obtained.

  13. Structural Basis for Crystal Sorption

    An intriguing property of some microporous metal-organic networks is that they can absorb large amounts of solvent molecules and increase in volume. Serre et al. (p. 1828) examined a series of compounds in which cobalt and iron centers linked by dicarboxylate organic linkers (fumarate, terephtalate, 2,6-naphthalene, and 4,4′-biphenyl) can exhibit very large volume changes (as high as 170%). By modeling x-ray powder diffraction data, they can describe movements within the framework that are driven by interactions between the guests and the skeleton; rotations of the linkers allow the structure to open up and increase in volume.

  14. Global Phytoplankton Model

    Complex ecosystems models tend to start with a minimum number of variables, partly because it is difficult to estimate the necessary parameters. Follows et al. (p. 1843) instead generated a global phytoplankton model, in which they begin with a large number of potential phytoplankton types with randomly varied ecophysiological parameters derived from lab and field studies. Within the model, competition for resources and the spatial variation of environmental constraints guide the establishment of regional ecosystems dominated by a few phytoplankton types. The biogeography is realistic, compared with real-world data taken from a north-south Atlantic transect.

  15. Definitive Telomerase Revealed

    Telomeres are specialized repeat structures at the very ends of linear eukaryotic chromosomes that protect the double-stranded DNA ends from recombining catastrophically with other regions of the genome. The protective telomere repeats are added onto the ends of the chromosomes by the enzyme telomerase, which has both protein and RNA components. The exact composition of telomerase has remained unclear because of its very low abundance. Now Cohen et al. (p. 1850) have isolated telomerase from immortalized cells, using the catalytic properties of the enzyme to effect its purification. Active telomerase consists of three components: reverse transcriptase, RNA, and dyskerin, a putative pseudouridine synthase. Indeed, dyskerin mutations are implicated in the disease dyskeratosis congenita, which has been attributed to telomerase deficiency.

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